Certain solids have a tendency to cake during storage and shipment. This problem is exemplified in the crop fertilizer field. Solid fertilizer materials are generally free-flowing granules that may be shipped in railcars or stored in large sheds. However, such materials can form firm cakes that can be broken only by severe impact. When caking occurs, the lumps or rocks that form cannot be easily handled by fertilizer spreading equipment. Fertilizer consumers often require a relatively uniformly sized material. A wide range of particle size forms when caked fertilizer is broken-up, effecting product quality.
Caking in rail cars may result in the formation of a layer (possibly several inches thick) of the fertilizer against the rail car walls requiring additional labor and longer unloading times to empty the railcars. Also, caking in storage piles results in longer loading times. Additional labor and special equipment may be needed to break-up the stored pile.
Ammonium sulfate is an excellent example of a fertilizer that tends to cake on sitting or during shipment. While not wishing to be bound by any theory, one hypothesis involving the tendency of ammonium sulfate (and other water soluble fertilizers) to cake involves the daily temperature/humidity cycle. During the evening as the temperature cools, moisture from the atmosphere or "dew" condenses. The ammonium sulfate in contact with condensed moisture is partially dissolved and forms "bridges" between ammonium sulfate particles. During the day, the condensed moisture is evaporated by the heat from the sun. The "bridges" are dried and harden. Additionally, the problem is aggravated because ammonium sulfate and other bulk fertilizers are generally stored and shipped in containers which are not air-tight. Finally, if the ammonium sulfate is not completely dried by the production process, the residual moisture contributes to the caking problem in the same way as condensed "dew".
The manufacture of ammonium sulfate is the subject of a large body of patent literature. For example, processes for making ammonium sulfate are described in U.S. Pat. No. 2,226,101 to Ogden. Ogden describes the addition of creosote or other oily substances to the mother liquor to carry impurities in the crystals to the surface of the liquor for removal, thus improving the whiteness of the crystals.
Ammonium sulfate is known to cake on standing in bulk. Methods proposed to overcome this tendency include crystal size and morphology control. Exemplary such methods are described in U.S. Pat. No. 1,919,707 to Gordon et al., U.S. Pat. No. 2,228,742 to Applebey and U.S. Pat. No. 5,330,544 to Thomson et al.
Sprays have been applied to ammonium sulfate crystals obtained from the dry distillation of coal to deodorize them. For example, Japanese Kokai 62(1987)-46920 describes spraying such crystals with a pH 7-8, ammonium-rich saturated ammonium sulfate solution.
Ammonium sulfate has also been granulated to improve particle size distribution. U.S. Pat. No. 4,277,253 to Walter et al. describes the granulation of ammonium sulfate and other fertilizer ingredients.
It is known to apply organic materials to such fertilizer granules to inhibit the tendency of the materials to cake. U.S. Pat. No. 4,717,555 to Newman et al. describes naphthalene sulfonates and water applied to ammonium salts to prevent caking and dust formation. U.S. Pat. No. 5,041,153 to Detroit describes lignosulfonate treated inorganic fertilizer chemicals that resist caking and dust formation.
Russian Inventor's Certificate 2019535 C1 describes the use of glycerol residuum (bottoms formed in the distillation of crude glycerol) applied to potassium chloride as an anti-dusting agent.
Lobeco Products Inc. offers an anti-caking product under the name Galoryl.TM.ATH 632. Galoryl ATH 632 is a solid at ambient temperature and must be heated (to about 80.degree. C.) to the liquid state before application. This adds to the handling difficulty and contributes to the safety precautions that must be in place to prevent bums from spillage of heated liquids. In addition to presenting handling difficulties, sprays such as Galoryl are expensive and add significantly to the production cost of free-flowing granules. Therefore, a need remains for safely and economically producing granules that are free-flowing even after storage and shipment.